PE2-5.3 Microfabrication & Packaging

What this is and why it exists

The first half of this topic is chip fabrication, because this field borrowed it whole. If that material is familiar, read it as revision.

The distinctly new content is micromachining. That is where material is deliberately removed to leave something free to move, and it is the moment a chip becomes a machine.

The vocabulary

• Photolithography — printing a pattern onto a light-sensitive layer.
• Epitaxy — growing a crystalline layer on a crystalline surface.
• Isotropic etch — one attacking equally in all directions.
• Anisotropic etch — one attacking some directions far faster than others.
• Bulk micromachining — etching into the substrate itself to form the structure.
• Surface micromachining — building structures on top and freeing them afterwards.
• Sacrificial layer — a layer placed only so that it can be dissolved later.
• Release — the step that dissolves that layer and frees the structure.
• LIGA — a process using deep X-ray exposure and electroplating to make tall structures.

The mental model

The shared steps come first. Patterning by light, doping by implantation, growing oxide, adding material from a gas or a target, and growing crystalline layers by epitaxy. Epitaxy earns particular attention here, because a crystalline layer has predictable mechanical properties and a deposited one does not.

One shared step also acquires a new significance. The stress locked into a deposited film decides whether a released structure stays flat or curls. Film stress is therefore a specification rather than a side effect.

Then etching, and this is where the field diverges. Etching can attack equally in every direction, or it can attack some directions far faster than others. In ordinary chip making that distinction is about feature size. Here it is what makes the whole field possible.

Bulk micromachining cuts into the wafer itself. Certain etches attack some crystal planes very much faster than others. The shape that results is decided by the crystal rather than by the tool. That is why bulk-machined cavities have characteristically sloped walls at a fixed angle. Once you know that angle comes from the crystal structure, the shapes stop looking arbitrary.

Surface micromachining works the other way. Build the structure on top of the wafer, with a sacrificial layer underneath it. Dissolve that layer and the structure is left standing free above the surface.

The release step is the whole point and the main hazard. It is the moment the device becomes mechanical. It is also where a structure most often fails. As the liquid dries, surface tension pulls a freed beam down onto the substrate. Once it touches, stiction may hold it there forever. Special drying methods exist for exactly this reason. A design that never lets a large flat area come close to the substrate avoids the problem entirely.

The two approaches shape everything else about a device. Bulk machining gives thick, strong structures made from the wafer's own single crystal. Surface machining gives thin structures with much more freedom of shape, made from deposited films with all the stress questions those bring.

LIGA answers a limitation both share. Neither reaches tall structures with vertical walls, because both are limited by how deep their processes can usefully go. LIGA exposes a thick layer with deep-penetrating X-rays and then electroplates metal into the resulting mould, which reaches aspect ratios the others cannot. It needs an X-ray source that few laboratories have, so it stays specialised and is used where nothing else works.

Packaging closes the topic and is harder here than for a chip, for the reason already met. The package must protect the device and simultaneously expose it to whatever it measures. It must also not squeeze it, because stress transmitted from the package into a sensing structure appears as a reading. Packaging is frequently the majority of the cost of the finished device.

What you should now be able to explain or do

Sequence the fabrication steps and say which are shared with ordinary chip making. Say why film stress becomes a specification in this field. Explain how an anisotropic etch produces shapes decided by the crystal. Compare bulk and surface micromachining on structure thickness, shape freedom and material. Explain the release step and why it is the usual point of failure.

Check yourself